Add RSpec formatter for Github Actions.

Library/Homebrew/dev-cmd/tests.rb

@@ -118,6 +118,8 @@ module Homebrew
         --out #{HOMEBREW_CACHE}/tests/parallel_runtime_rspec.log
       ]
 
+      bundle_args << "--format" << "RSpec::Github::Formatter" if ENV["GITHUB_ACTIONS"]
+
       unless OS.mac?
         bundle_args << "--tag" << "~needs_macos" << "--tag" << "~cask"
         files = files.reject { |p| p =~ %r{^test/(os/mac|cask)(/.*|_spec\.rb)$} }

Library/Homebrew/test/sandbox_spec.rb

@@ -2,7 +2,7 @@
 
 require "sandbox"
 
-describe Sandbox do
+describe Sandbox, :needs_macos do
   define_negated_matcher :not_matching, :matching
 
   let(:dir) { mktmpdir }

Library/Homebrew/test/spec_helper.rb

@@ -36,6 +36,7 @@ $LOAD_PATH.push(File.expand_path("#{ENV["HOMEBREW_LIBRARY"]}/Homebrew/test/suppo
 require_relative "../global"
 
 require "test/support/no_seed_progress_formatter"
+require "test/support/github_formatter"
 require "test/support/helper/cask"
 require "test/support/helper/fixtures"
 require "test/support/helper/formula"

Library/Homebrew/test/support/github_formatter.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+require "rspec/core"
+require "rspec/core/formatters/base_formatter"
+
+# TODO: Replace with `rspec-github` when https://github.com/Drieam/rspec-github/pull/4 is merged.
+module RSpec
+  module Github
+    class Formatter < RSpec::Core::Formatters::BaseFormatter
+      RSpec::Core::Formatters.register self, :example_failed, :example_pending
+
+      def self.relative_path(path)
+        if (workspace = ENV["GITHUB_WORKSPACE"])
+          workspace = "#{File.realpath(workspace)}#{File::SEPARATOR}"
+          absolute_path = File.realpath(path)
+
+          return absolute_path.delete_prefix(workspace) if absolute_path.start_with?(workspace)
+        end
+
+        path
+      end
+
+      def example_failed(failure)
+        file, line = failure.example.location.split(":")
+        file = self.class.relative_path(file)
+        output.puts "\n::error file=#{file},line=#{line}::#{failure.message_lines.join("%0A")}"
+      end
+
+      def example_pending(pending)
+        file, line = pending.example.location.split(":")
+        file = self.class.relative_path(file)
+        output.puts "\n::warning file=#{file},line=#{line}::#{pending.example.full_description}"
+      end
+    end
+  end
+end